The present invention relates to electrical circuit means for maintaining cross-talk balance, in an apparatus including a demodulation system phased to an AC power supply subject to spurious frequency fluctuations. One particular application of said circuit means is in a double-beam ratio-recording spectrophotometer.
In a spectrophotometer of the type referred to, the radiation emerging from a photometric source is passed through a sample under analysis sited at a sample station and reference material sited at a reference station. The radiation emerging from the sample and the reference is alternately directed into a common path by a constant speed beam recombining means, such as a sector mirror, and after passing through a monochromator is made to impinge upon a detector which provides an electrical signal output the amplitude of which corresponds to the intensity of the impinging radiation. A very simple, conventional way of driving a beam re-combining sector mirror at constant speed is to couple it to a synchronous AC motor energized from the public AC supply.
The dector output cannot, naturally, discriminate between the signal components due detector the radiation that has passed through the sample and the reference, respectively. In order to separate the two components a demodulation circuit is required. Now, the generation of the two components is strictly related in time to the speed of the sector mirror and consequently to the frequency of the AC supply. In other words, the duration of the time intervals during which the detector sees the sample and the reference, respectively, is related to the frequency of the AC supplies. The time must obviously be compressed if the frequency should rise above its nominal value, and be extended if the frequency should fall below said value.
Photometric detectors used in spectrophotometry, and in particular detectors normally used in infra-red spectrophotometers, exhibit a significant time lag between the optical signal impinging thereon and the rise of the electrical signal to its peak value. Reference to this will be made later in greater detail, but it will suffice to say here that the timing of the demodulation must take this fact into account in order to utilize the available signal output to best advantage and, in general, each demodulation point must be chosen to occur some time after the start of a radiation pulse.
The time lag referred to gives rise to cross-talk between the sample signal and the reference signal in both the demodulated sample signal and the demodulated reference signal. This crosstalk may be minimized by suitably selecting the timing of the demodulation points so as to ensure that all the signal areas due to the reference removed by demodulation from the demodulated sample signal are substantially identical and all the signal areas due to the sample similarly removed from the reference signal are identical, so that the two demodulated signals may then be ratioed together to yield an electrical value truly representative of the ratio between the optical transmission through the sample and that through the reference. When crosstalk has been minimized, it may be said that cross-talk balance has been established.
We have found that in the use of a prior art ratio-recording spectrophotometer the trace produced on the recording chart by the recording pen occasionally presented a modulation which could not be attributed to the nature of the sample. Upon investigation we discovered that it was due to spurious fluctuations in the nomimal frequency value of the AC supply. We assumed that it was due to the fact that the consequent compression and expansion of the time during which the detector was exposed to an optical pulse, alternately from the sample and the reference, and the strict phasing of the demodulation points to the AC supply meant that the value attained by the detector signal at each demodulation point varied as the frequency varied, with the result that photometric accuracy was impaired in the manner revealed by the recorded trace. Again, a detailed discussion of this will be given later.
The fluctuations in the frequency of the public AC supply with which the present invention is concerned are the comparatively slow fluctuations within the range 10 to 15 percent of the nominal value. In other words, the invention deals with fluctuations the duration of which is related to the time constant of the generators at the power station. These generators have a considerable moment of inertia and this means that in normal use, when constant speed operation is aimed at, they accelerate and decelerate very slowly, as a result, for example, of changes in load. It has been found that comparatively fast frequency changes of a transient nature do occur in a public AC supply, but these are too small to cause any concern. As a general criterion, the present invention is concerned with fluctuations occuring over tens of demodulation cycles.
An example of a ratio recording double-beam spectrophotometer to which the present invention is applicable is disclosed in the U.S. Pat. No. 3,542,480, which is particularly directed to the problem of eliminating the effect of re-radiation in the sample and reference signals, respectively. In that patent, no reference is made to cross-talk balance but the demodulation system described therein would take care of it, as in other prior art arrangements, along the lines referred to above, although it would in no way be able to combat the effect of spurious frequency fluctuations on photometric accuracy, with which the present invention is specifically concerned.
Ratio-recording spectrophotometer incorporating the photometric system disclosed in the United States patent referred to will be later described as an example of one embodiment of the present invention. That photometric system is not essential to the realization of the present invention in its application to said spectrophotometer but it has been chosen for the purposes of the detailed description because it represents an advanced prior art system the advantages of which are retained when it is incorporated in a spectrophotometer in accordance with the present invention.
It is therefore a principal object of the present invention to provide circuit means to maintain cross-talk balance in an apparatus comprising a demodulation system phased to an AC power supply. It is a further object of the invention to provide circuit means to maintain cross-talk balance in the demodulation system of a ratio-recording spectrophotometer.
In achieving these and other objectives, the present invention includes a signal generating arrangement for producing at a generating frequency subject to spurious fluctuations an electrical signal having a first component varying in response to a first quantity and a second component varying in response to a second quantity. It further includes a demodulating means referenced to said signal generating means for switching each component into its own demodulation channel, and a phasing means forming part of said demodulating means for adjusting the timing of the demodulation switching referred to said generation frequency so as to separate the two components while substantially establishing cross-talk balance between the two demodulation channels. Means effective on said timing are operatively associated with the demodulating means to minimize the effect of spurious fluctuations of said frequency on said cross-talk balance.
In one specific way of carrying the invention into effect the phasing means are associated with demodulation command means for producing before the rise of a signal component a demodulation command correlated to said timing and the demodulating means include demodulation execution means for switching said signal component into its allotted demodulation channel after said rise. The means effective on said timing comprise a time delay setting means operationally related to both the demodulation command means and the demodulation execution means for delaying the operation of the demodulation execution means by a predetermined time interval from the occurrence of said demodulation command. The invention is described way of example, in its application to a ratio-recording spectrophotometer in which the outputs of the demodulating channels are ratioed and the signal representing the ratio between sample transmission (or absorption) and reference transmission (or absorption) is utilized as an input signal to a chart recorder.